Abstract
The purpose of this study was to evaluate whether it is possible to restore the high-frequency angular vestibulo-ocular reflex (aVOR) in patients suffering from a severe bilateral vestibulopathy (BV) and implanted with a vestibular implant prototype. Three patients (S1–3) participated in the study. They received a prototype vestibular implant with one to three electrode branches implanted in the proximity of the ampullary branches of the vestibular nerve. Five electrodes were available for electrical stimulation: one implanted in proximity of the left posterior ampullary nerve in S1, one in the left lateral and another one in the superior ampullary nerves in S2, and one in the right lateral and another one in the superior ampullary nerves in S3. The high-frequency aVOR was assessed using the video head impulse test (EyeSeeCam; EyeSeeTec, Munich, Germany), while motion-modulated electrical stimulation was delivered via one of the implanted vestibular electrodes at a time. aVOR gains were compared to control measurements obtained in the same patients when the device was not activated. In three out of the five tested electrodes the aVOR gain increased monotonically with increased stimulation strength when head impulses were delivered in the plane of the implanted canal. In these cases, gains ranging from 0.4 to values above 1 were measured. A “reversed” aVOR could also be generated when inversed stimulation paradigms were used. In most cases, the gain for excitatory head impulses was superior to that recorded for inhibitory head impulses, consistent with unilateral vestibular stimulation. Improvements of aVOR gain were generally accompanied by a concomitant decrease of corrective saccades, providing additional evidence of an effective aVOR. High inter-electrode and inter-subject variability were observed. These results, together with previous research, demonstrate that it is possible to restore the aVOR in a broad frequency range using motion-modulated electrical stimulation of the vestibular afferents. This provides additional encouraging evidence of the possibility of achieving a useful rehabilitation alternative for patients with BV in the near future.
Highlights
Every day we are confronted to a variety of dynamic situations where precise head and body motion information are required to guarantee adequate function, safety, and well-being
Our results demonstrate that motion-modulated electrical stimulation of the ampullary branches of the vestibular nerve can be an effective means of restoring this reflex
The video head impulse test (vHIT) proved to be suitable for this assessment, achieved for the first time in human patients suffering from bilateral vestibulopathy (BV)
Summary
Every day we are confronted to a variety of dynamic situations where precise head and body motion information are required to guarantee adequate function, safety, and well-being. One of the major functions of the vestibular system is the generation of compensatory eye movements during head motion to achieve gaze stabilization. Gaze stabilization is mainly achieved with the angular vestibuloocular reflex (aVOR), which is one of the key functions of the semicircular canals. The high-frequency aVOR can be quantified by applying brisk, passive, and unpredictable head rotations (i.e., head impulses) in the plane of each semicircular canal while the tested subject fixates a visual target. A compensatory eye movement is generated in the opposite direction from that of the head impulse by the aVOR. The clinical observation of these compensatory saccades after head impulses has been used as a marker of semicircular canal dysfunction for several decades [head impulse test (HIT); [1]]. The vHIT has become the gold standard in vestibular testing in the high-frequency domain [3,4,5]
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